The present invention pertains to drag-type drill bits. In such a bit, a plurality of cutting members may be mounted on a bit body. Typically, each such cutting member comprises an elongate or stud-like body, e.g. of sintered tungsten carbide, carrying a layer of superhard material, e.g. polycrystalline diamond, which defines the actual cutting face. Such use of layers of different materials renders the cutting members self-sharpening in the sense that, in use, the tungsten carbide material will tend to wear more easily than the polycrystalline diamond material. This causes the development of a small step or clearance at the juncture of the two materials so that the earth formation continues to be contacted and cut substantially only by the edge of the diamond layer, the tungsten carbide substrate having little or no high pressure contact with the earth formation. Because the diamond layer is relatively thin, the edge thus maintained is correspondingly sharp.
The bit bodies in which these cutting members are mounted may generally be divided into two types: bodies formed of steel or similar ductile metallic material, and bodies formed of tungsten carbide matrix material. With steel body bits, it is relatively easy to mount the cutting members in the bit body by interference fitting techniques, e.g. press fitting or shrink fitting. In some instances, tungsten carbide matrix body bits are preferred over steel body bits because of their hardness. However, although harder than steel and similar metals, tungsten carbide matrix is also more brittle, rendering interference fitting techniques more difficult. Accordingly, in matrix body bits, the cutting members are often brazed into place.
A problem commonly associated with the use of such bits is that of selecting a suitable back rake angle for a particular drilling job. It has been found that the effectiveness of the cutting members and the bit in general can be improved by proper arrangement of the cutting members and, more specifically, their cutting faces, with respect to the body of the drill bit, and thus to the earth formation being cut. Conventional cutting faces are typically planar (although outwardly convex cutting faces are known). The cutting members can be mounted on the bit so that such planar cutting faces have some degree of side rake and/or back rake. Any given drill bit is designed to cut the earth formation to a desired three-dimensional "profile" which generally parallels the configuration of the operating end of the drill bit. "Side rake" can be technically defined as the complement of the angle between (1) a given cutting face and (2) a vector in the direction of motion of said cutting face in use, the angle being measured in a plane tangential to the earth formation profile at the closest adjacent point. As a practical matter, a cutting face has some degree of side rake if it is not aligned in a strictly radial direction with respect to the end face of the bit as a whole, but rather, has both radial and tangential components of direction. "Back rake" can be technically defined as the angle between (1) the cutting face and (2) the normal to the earth formation profile at the closest adjacent point, measured in a plane containing the direction of motion of the cutting member, e.g. a plane perpendicular to both the cutting face and the adjacent portion of the earth formation profile (assuming a side rake angle of 0.degree.). If the aforementioned normal falls within the cutting member, then the back rake is negative; if the normal falls outside the cutting member, the back rake is positive. As a practical matter, back rake can be considered a canting of the cutting face with respect to the adjacent portion of the earth formation profile, i.e. "local profile," with the rake being negative if the cutting edge is the trailing edge of the overall cutting face in use and positive if the cutting edge is the leading edge. Substantial positive back rake angles have seldom, if ever, been used on the type of bit in question. Thus, in the terminology of the art, a negative back rake angle is often referred to as relatively "large" or "small" in the sense of its absolute value. For example, a back rake angle of -20.degree. would be considered larger than a zero back rake angle, and a back rake angle of -30.degree. would be considered still larger.
Proper selection of the back rake angle is particularly important for most efficient drilling in a given type of earth formation. In soft formations, relatively small cutting forces may be used so that cutter damage problems are minimized. It thus becomes possible, and indeed preferable, to utilize a very slight negative rake angle, a zero rake angle or even a slight positive rake angle, since such angles permit fast drilling and optimize specific energy. However, in hard rock, it is necessary to use a significant negative rake angle, in order to avoid excessive wear in the form of breakage or chipping of the cutting members due to the higher cutting forces which become necessary.
Problems arise in drilling through stratified formations in which the different strata vary in hardness, as well as in drilling through formations which, while substantially comprised of relatively soft material, contain "stringers" of hard rock. In the past, one of the most conservative approaches to this problem was to utilize a relatively large negative back rake angle, e.g. -20.degree. for the entire drilling operation. This would ensure that, if or when hard rock was encountered, it would be drilled without damage to the cutting members. However, this approach is unacceptable, particularly where it is known that a substantial portion, specifically the uppermost portion, of the formation to be drilled is soft, because the substantial negative back rake angle unduly limits the speed of drilling in the soft formation.
Another approach, applicable where the formation is stratified, is to utilize a bit whose cutting members have relatively small or zero back rake angles to drill through the soft formation and then change bits and drill through the hard formation with a bit whose cutting members have substantial negative back rake angles, e.g. -20.degree. or more. This approach is unsatisfactory because of the time and expense of a special "trip" of the drill string for the purpose of changing bits.
If it is believed that the formation is uniformly soft, a somewhat daring approach is to utilize the relatively small back rake angles in order to maximize the penetration rate. However, if a hard stringer is encountered, catastrophic failures can result. For example, severe chipping of only a single cutting member increases the load on neighboring cutting members and shortens their life resulting in a premature "ring out," i.e. a condition in which the bit is effectively inoperative.
Still another problem associated with the general type of bit and cutting member described above, is that chips of the formation material being drilled may build up ahead of the cutting faces of the cutting members.